Considering, by way of example, a conventional thermo-electric plant, we observe that each of its generating set comprises a (gas or steam) turbine and a synchronous alternator.
The alternator converts the mechanical energy produced by the turbine into electrical energy to be delivered, in general, on the national electrical supply net.
Both the turbine and the alternator are operated under the control of their respective automatic regulating system:                the speed/load regulator for the turbine,        the voltage regulator for the alternator.        
The voltage regulator for the alternator, often identified with the acronym AVR (Automatic Voltage Regulator), mainly serves the function of automatically regulating the electrical stator quantities of the alternator (voltage, reactive power, power factor cos φ),
Moreover, a compensator or stabiliser device PSS (Power System Stabiliser) is associated with the voltage regulator AVR and generates stabilising signals to be provided to the regulator, such as to have a relevant role for limiting problems relating to the known phenomenon of local electromechanical swings.
This PSS stabiliser serves the function of correcting, by means of the generated stabilising signals, the excitation of the synchronous alternator G with appropriate transient compensating pulses which, delivered at determined instants during load variations thereof (e.g. load connections and disconnections), reduce and dampen the consequent electromechanical swings of the turbine-alternator arrangement.
Today, the standards that set the operating specifications of energy production plants are particularly strict on the damping of electromechanical swings. In other words, such standards require the delivered electrical power to be stabilised and, hence, the electromechanical swings to be dampened in short times and after a few cycles (e.g. after 3-4 oscillations).
According to conventional technologies, the “PSS” function acts based on the variations of its input quantities, which are active power (PE) and frequency (f), currently measured at the alternator terminals.
One limitation of these conventional compensator devices is that their correct operation requires an accurate knowledge of the physical-mathematical model of the process to be controlled, and that their optimisation is centred only on a narrow working area of the alternator.
Since an accurate knowledge of the process parameters is very difficult, especially when they change over time, conventional compensator device cannot provide sufficient and optimised performance.
Moreover, control laws of prior art compensator devices are based on a high number of both parameters and of possible combinations thereof.
This also makes yet more critical and particularly complex all calibration operations, which, therefore, must essentially be based on the experience and sensitivity of the commissioning operator.